The present disclosure relates generally to imaging using interferometry. More particularly, the present disclosure pertains to the use of methods and systems using optical coherence tomography (OCT) for various biological tissue imaging applications including, e.g., tumor detection (e.g., early detection), dental caries detection (e.g., early detection), wounds and/or burnt tissue imaging, etc. Further, such methods and systems using optical coherence tomography may be useful in the following fields: ophthalmology, surgery, cardiology, neurology, gastroenterology, and/or dermatology.
OCT has established itself as an important imaging modality for numerous medical and biological applications. Generally, OCT is a sub-surface imaging technique that uses either a low-coherence light source (time-domain systems) or a swept source (frequency-domain systems). OCT has about one to about two orders of magnitude higher resolution than ultrasound imaging and an imaging depth of about 4 to about 6 millimeters. Further, OCT may be implemented using an optical fiber based probe that is mounted on a syringe to image internal body organs (see, e.g., Y. Mao, S. Chang, S. S. Sherif, and C. Flueraru, “Graded-index fiber lens proposed for ultrasmall probes used in biomedical imaging,” Applied Optics, 46, pp. 5887-5894 (2007)).
The use of light in OCT techniques may be safer to most biological samples than ionizing radiation like X-rays or gamma rays, and further may allow for spectroscopic characterization of an object, e.g., to detect tumor in tissue (see, e.g., Chengyang Xu, Claudio Vinegoni, Tyler S. Ralston, Wei Luo, Wei Tan, and Stephen A. Boppart, “Spectroscopic spectral-domain optical coherence microscopy,” Opt. Lett. 31, 1079-1081 (2006)).
FIG. 1 depicts the image resolution and penetration depth for different biomedical imaging modalities. As shown, a tradeoff may exist between the imaging resolution and penetration depth in sub-surface biomedical imaging applications. In addition to the relatively low cost of OCT, another advantage may be its combination of imaging resolution and penetration depth that may be unattainable by other biomedical imaging modalities.
Further, the ability of OCT to detect microscopic changes in the morphology and composition of tissue may make OCT an ideal non-invasive optical biopsy method for the early detection of cancer (see, e.g., B. E. Bouma and G. J. Tearney, eds., Handbook of Optical Coherence Tomography (Marcel Dekker, New York, 2001)).
Many OCT imaging models only consider single scattered photons (e.g., ballistic photons) inside an object as contributors to the imaging process. While ballistic-photon-based mathematical OCT models may simplify the analysis, it has been demonstrated that multiple-scattered photons, or multiple-scattered light, which are dominant at large depths, may be a fundamental limitation in increasing the imaging depth of OCT in tissue (see, e.g., M. J. Yadlowsky, J. M. Schmitt, and R. F. Bonner, “Multiple scattering in optical coherence microscopy,” Appl. Opt. 34, 5699-5707 (1995)). Recent application of OCT imaging for the early detection of breast cancer may represent an area where increased imaging depth may be useful (see, e.g., A. M. Zysk and S. A. Boppart, “Computational methods for analysis of human breast tumor tissue in optical coherence tomography images,” Journal of Biomedical Optics 11 (5), 054015 (2006)).
When an optical field is incident on biological tissue, the optical field penetrates the tissue to a depth that depends on the optical properties of biological tissue and the intensity of the field. Biological tissue is typically an inhomogeneous optical medium, and therefore, part of the optical field is reflected. The light that is scattered once inside the tissue is called the single-scattered photons or light; the light that is scattered more than once inside the tissue is called multiple-scattered photons or light. In other words, light that is only scattered by one localized scattering center is called single-scattered light, and light that is scattered by more than one localized scattering center grouped together is known as multiple scattered light.
In OCT imaging, backscattered light, which is the light reflected back to where it came from, may be used to reconstruct the subsurface structure of an object, even though the detected light has both single and multiple-scattered components. As imaging depth increases, the ratio of detected single to multiple-scattered components decreases. As such, multiple scattering of light introduces a loss of contrast in OCT images, reduces the imaging axial resolution, and decreases the imaging depth in biological tissue (see, e.g., Schmitt, J. M., “Optical coherence tomography (OCT): a review,” IEEE Journal of Selected Topics in Quantum Electronics 5(4), 1205-1215 (1999); Schmitt, J. M., and Knüttel, A., “Model of optical coherence tomography of heterogeneous tissue,” Journal of the Optical Society of America A 14(6), 1231-1242 (1997); and Fercher, A. F., Drexler, W., Hitzenberger, C. K., and Lasser, T., “Optical coherence tomography-principles and applications,” Reports on Progress in Physics 66, 239-303 (2003)).
Several researchers have studied multiple-scattered light in biological tissue to characterize its effects on OCT imaging. For example, Yadlowsky et al. has experimentally classified the directions of multiply scattered light into small and wide angles (see, e.g., Yadlowsky, M. J., Schmitt, J. M., and Bonner, R. F., “Multiple scattering in optical coherence microscopy,” Applied Optics 34(25), 5699-5707 (1995)). It was demonstrated that multiple-scattered light with small angles may contribute to backscattered light and may enhance the reflectance of small structures of biological tissue. Further, it was also demonstrated that multiple-scattered light with wide angles may reduce the contrast of subsequent features, and blurred and produced broad haze in OCT images.
Further, for example, Adie et al. quantified multiple-scattering of light using a polarization sensitive OCT system (see, e.g., Adie, S. G., Hillman, T R., and Sampson, D. O., “Detection of multiple scattering in optical coherence tomography using the spatial distribution of Stokes vectors,” Optics Express 15(26), 18033-18049 (2007)). It was demonstrated that the correlation of both local polarization states of the backscattered light may characterize the relative presence of both the multiple-scattered and the single-scattered light from biological tissue.
Each of the methods presented by Yadowlsky et al. and Adie et al. are empirical in nature and were performed for specific biological tissue types using time-domain optical coherence tomography rather than swept-source optical coherence tomography.
Further, an ad-hoc attempt to perform time-domain OCT in the presence of continuous-wave (CW) ultrasound in tissue has been previously discussed (see, e.g., J. O. Schenk and M. E. Brezinski, “Ultrasound induced improvement in optical coherence tomography (OCT) resolution,” Proceedings of the National Academy of Sciences, 99, pp. 9761-9764 (2002)). In this ad-hoc attempt, a 7.5-MHz ultrasound transducer was placed approximately in parallel to the OCT beam, and the ultrasound beam was brought into direct contact with the tissue with ultrasound transducing medium. The OCT imaging was performed with the ultrasound beam in three settings: ultrasound off, pulsed ultrasound, and CW ultrasound. The CW ultrasound was performed at a power of 10.6 milliwatts (mW), beam diameter of 0.15 centimeters (cm), and focal length of 2.1 cm. The pulsed ultrasound was performed with an average power of 17.8 mW, beam diameter of 0.24 cm, and a focal length of 2.1 cm. Further, the ultrasound beam was used over 1.5 cm proximal to the focus, resulting in an essentially collimated beam, and the pulse repetition rate was 1.3 milliseconds (ms) with an average pulse intensity of 225 Watts per cm squared.
As reported in this article (i.e., “Ultrasound induced improvement in optical coherence tomography (OCT) resolution”), the presence and absence of a CW ultrasound beam is apparent. More specifically, it can be seen that the noise between scatterers may be reduced in the presence of ultrasound deep within the tissue. The article generally concluded that combining OCT with a parallel ultrasound beam may result in an improvement in resolution through a reduced effect of multiple scattering due to photon-phonon interaction. The techniques described in the article, however, only utilized Time-Domain OCT, and further, the OCT algorithms did not take into consideration, or account for, the effects of the CW ultrasound within the tissue in the processing of imaging data and/or constructing of an image from such imaging data.
An integrated computational imaging system (also referred to as a hybrid imaging system) combines a modified optical imaging system and a digital post-processing step. An integrated computational imaging system is different from a system obtained by cascading a physical imaging system and a digital image processing step. In an integrated computational system, both the physical imaging and digital modules are parts of a single system and the imaging process is divided between them. Thus, in an integrated computational system, the final image is obtained by digitally processing an intermediate detected image. The design flexibility of an integrated computational system could be used to achieve imaging performance that would be otherwise unattainable by any similar conventional system (see, e.g., W. T. Cathey and E. R. Dowski, “New paradigm for imaging systems,” Appl. Opt., 41, pp. 6080 (2002)).
One example of an integrated computational imaging system may be described in U.S. Pat. No. 7,031,054 B2 entitled “Methods and Systems for Reducing Depth of Field of Hybrid Imaging Systems” issued on Apr. 18, 2006 to Cathey, Jr. et al., which is hereby incorporated herein by reference.